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I recently had a Buderus G115 boiler installed, with a Buderus ST-150 40 gallon indirect hot water tank. The aquastat for the hot water tank is a Goldline SP-30D, one of the controls recommended by Buderus. The zone valve is a Taco 1 inch. The goal was to get 120 degree DHW. The DHW was running hot compared to the upper limit set on the aquastat, so we lowered the high temp to 105 degrees, with a 5 degree differential. (With this setting, the stat would call for heat to come to the tank at 100 degrees, and SHOULD stop calling at 105 degrees.) With this setting, the temp goes as high as 140 degrees and never lower than 120 degrees. Many times it is 130 degrees (all temps measured at the tap in the kitchen).
Can anyone explain why this is happening? The aquastat appears to be wired correctly and all the jumpers are set correctly. It was installed by a pro, and he is somewhat stumped by this. Perhaps the sensor is bad, but it is pretty close, because when the temp in the tank is 130, the stat won't go on until you turn the dial past 130. Is the Taco zone valve not closing, thus allowing hot water to continue to circulate in the zone? Any insight would be greatly appreciated!

Do you not have a tempering valve on the output of the DHW? I have a Buderus G115 with the ST-150 as well. The difference is that I run the Tank at 140 degrees to avoid as issue with bacteria and disease. The comment below was pulled form the america health journal:

There is also concern for increased risk of bacteria growth in water storage systems with such tepid water temperatures below 120 degrees ((ASTM F-444.88). Patrick J. Higgins, a master plumber and plumbing consultant in Frederick, Maryland, cautions that water heaters set below 120°F pose a health risk by inviting the growth of bacteria, including the one that causes Legionnaire's disease

the way around this is to set the temp to 140 and use a tempering valse to bring it down to 104 for most faucets.

Sorry I could not help ith the aquastat issue, I use the buderus logamatic controller to control my DHW tank, but I thought that you might find the article interesting on a low temp seeting in the tank.

When another heating zone (probably the longest zone)is calling and the indirect water heater isn't calling, put your hands on the indirect zone piping and see if its heating up. If it is either the indirect storage tanks' zone valve is slightly stuck open or it is being lifted open by the circulator.
Then you either need the zone valve replaced or piped differently.
All circulators are better but more expensive.

It would probably save a lot of time if he just installed a seperate circulator for the indirect piped close to the boiler. This is the correct way to do it. The supply and return pipng should be the first zones off the headers.
The circulator should be low near the storage tank. And a check valve to prevent overheating.

That tank is not designed to put out 120 or so degrees. It should run at 150 degrees, a mixing valve should be installed to temper it to what your needs are, like 120. You get a much more even temprature and your hot water will last much longer. You should also have flow checks on all zones to keep heating water from migrating when there is no call for hot water. When I install these I like to use a Taco zone control to make the system run priority hot water

He will be installing a separate circulator and a tempering valve next week, at no extra charge. I am pleased that he is willing to make it right. He provided a lot of little "extras" at no charge, such as separating my first and second floor into separate zones. Perhaps it should have been better up front, but one sign of a "pro" is the willingness to make the job right if mistakes were made.

I'm surprised it wasn't done this way to begin with. I am pleased to hear that he is doing it at no charge. It sounds like he got caught trying to cut corners. Most customers probably wouldn't have been as perceptive as you. your contractor is doing the right thing by fixing his boo boo for 0$. Let us know how the system works after mods have been done.

Update: They installed a Honeywell tempering valve off my Buderus hot water tank, as well as the separate circulator to supply the tank. The valve seems to work OK, but there is a complication. When the valve is set at 120 degrees (the max it can go), the water at the tap is 140 degrees for a while, then it drops down to 120. Looking at it further, I noticed that the cold water side of the valve gets very hot once the hot water is turned off. The hot water is migrating through the tempering valve and over into the cold water supply line. In a short time, the cold water supply line to the tank is very hot and the main supply to the whole system starts to warm up. Once the hot water is turned on, the cold ends up taking over and the mixing works OK. My concern is the wasting of hot water as it heats up the cold water pipe, not to mention the HOT initial water at 140 when it first comes on. I then pulled out the manual for the Honeywell tempering valve that was left after the install. There it clearly shows that a flow check valve should have been installed on the cold side. One was not installed on mine. I'm just curious: Is it a standard item to have a flow check valve installed with a tempering valve?

You should have a domestic water expansion tank on the cold side, like an Amtrol ST5. Most cities have a check valve at your water meter already. But if not they may require it at the storage tank. If you have a well then its up to you if you want it or not.
But if it gets installed with no expansion tank the temperature and pressure relief valve on the storage tank will spit.